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[Babesia divergens] [Babesia duncani] [Babesia microti] [Babesia MO-1]
Diagnosis can be made by microscopic examination of thick and thin blood smears stained with Giemsa. Repeated smears may be needed.
Diagnosis of Babesia infection should be made by detection of parasites in patients' blood smears. However, antibody detection tests are useful for detecting infected individuals with very low levels of parasitemia (such as asymptomatic blood donors in transfusion-associated cases), for diagnosis after infection is cleared by therapy, and for discrimination between Plasmodium falciparum and Babesia infection in patients whose blood smear examinations are inconclusive and whose travel histories cannot exclude either parasite.
The indirect fluorescent antibody test (IFA) using B. microti parasites as antigen detects antibodies in 88-96% of patients with B. microti infection. IFA antigen slides are prepared using washed, parasitized erythrocytes produced in hamsters. Patients' titers generally rise to ≥1:1024 during the first weeks of illness and decline gradually over 6 months to titers of 1:16 to 1:256 but may remain detectable at low levels for a year or more. Specificity is 100% in patients with other tick-borne diseases or persons not exposed to the parasite. Cross-reactions may occur in serum specimens from patients with malaria infections, but generally titers are highest with the homologous antigen.
The extent of cross-reactivity between Babesia species is variable. A negative result with B. microti antigen for a patient exposed on the West Coast may be a false-negative reaction for Babesia infection. Individuals whose exposure could have occurred on the West Coast should be tested also for antibodies to the Babesia duncani, because of the lack of cross-reactivity with B. microti.
Krause PJ, Telford S RI, Ryan R, et al. Diagnosis of babesiosis: Evaluation of a serologic test for the detection of Babesia microti antibody. J Infect Dis 1994;169:923-926.
In some infections with intraerythrocytic parasites, the morphologic characteristics observed on microscopic examination of blood smears do not allow an unambiguous differentiation between Babesia and Plasmodium. Moreover, potential blood donors may have subclinical symptoms and very low parasitemia, undetectable in blood smears. In such cases, the diagnosis can be derived from molecular techniques, such as PCR using the appropriate primers and single-step, or the more sensitive nested PCR technique. In addition, molecular approaches are very valuable in investigations of new Babesia variants (or species) observed in recent human infections in the US and in Europe.
A: Agarose gel (2%) analysis of a PCR diagnostic test for detection of Babesia microti DNA. PCR was performed using a nested protocol with primers BAB1 and BAB41 (first round) and BAB2 and BAB3 (second round).1
- Lane S: Molecular base pair standard (50-bp ladder). Black arrows show the size of standard bands.
- Lane 1: First step amplification with primers BAB1 and BAB41 of the nested PCR protocol for detection of B. microti in DNA extracted from whole blood. The specimen, serologically positive for B. microti, was submitted to CDC by the American Red Cross. The red arrow shows the single-step PCR diagnostic band for B. microti (size: 238 bp).
- Lane2: Nested PCR with primers BAB2 and BAB31 using as template the product of the first step amplification. The blue arrow shows the nested PCR diagnostic band for B. microti (size: 154 bp). Please note the enhanced sensitivity of B. microti DNA detection with the nested reaction.
Persing DH, Mathiesen D, Marshall WF, Telford SR, Spielman A, Thomford JW, Conrad PA. Detection of Babesia microti by polymerase chain reaction. J Clin Microbiol 1992;30:2097-103.